1. Field of the Invention
The present invention generally relates to plasma processing. More particularly, the present invention relates to a radio frequency (RF) plasma source for use in plasma material processing.
2. Background of the Related Art
Plasma material processes are widely used in the fabrication of integrated circuits on semiconductor substrates. These processes typically include etching, chemical vapor deposition, physical vapor deposition and other vacuum processes. During these processes, the semiconductor substrates are exposed to a gaseous plasma within a vacuum processing chamber. Radio frequency energy (RF between 3-30 MHz), typically at 13.56 MHz, is used to excite a processing gas that is supplied to the processing chamber and generate a plasma. The plasma may be generated within the processing chamber and/or introduced from a remote plasma generator to the processing chamber. Plasma generation within the processing chamber and remote plasma generation are both well known in the art. Each method of plasma generation has been utilized in a variety of plasma material processes. For example, remote plasma generation of a cleaning gas, such as NF.sub.3, has been successfully utilized to clean deposition chambers or process kit components (e.g., gas distributors, clamp rings, etc.) made of ceramic or aluminum.
FIG. 1 is a simplified schematic view of a typical remote inductively coupled plasma source. The plasma source 100 generally comprises a tube 102, a coil 104 spirally wound outside and along the length of the tube 102 and an RF power source 106 connected to the coil 104. Generally, inductive coupling, as shown in FIG. 1, is preferred over capacitive coupling because the plasma density generated by inductive coupling for a given power is higher than that generated by capacitive coupling with the same power. The higher plasma density generally results in an increased reaction rate, shorter processing time and higher throughput. The RF power source 106 supplies to the coil 104 the RF energy needed to generate a plasma within the tube. Typically, an RF match network 108 is connected between the RF power source 106 and the coil 104 to provide an impedance match between the RF power source 106 and the coil 104. The impedance match ensures that the RF power supplied to the coil 104 is not reflected back to the RF power source 106 and provides optimal power transfer between the RF power source 106 and the coil 104. The tube 102 includes a process gas inlet 110 disposed on one end and a plasma outlet 112 disposed on the other end. The process gas inlet 110 is fluidly connected to a processing gas source (not shown), and the plasma outlet 112 is fluidly connected to a processing chamber (not shown). The remote plasma source 100 is generally mounted on a surface of the chamber enclosure, typically on top of a lid to the chamber enclosure.
During processing, the processing gas is introduced into the tube 102 through the process gas inlet 110, and the RF power source 106 is activated to supply an RF power to the coil 104. The RF power energizes the coil 104 and produces an RF field within the tube 102 that excites the processing gases to a plasma state. The plasma then flows out of the plasma outlet 112 into the processing chamber. Typically, the processing gas is continuously introduced into the tube 102 and excited to a plasma to provide a continuous plasma supply into the processing chamber throughout the processing period.
Typically, the RF power source supplies a high peak-to-peak voltage on the order of a few kilo volts to one end of the coil 104 while the other end of the coil 104 is grounded. A problem with the inductive RF coil having one end grounded and the other end connected to a high voltage is that the high RF peak-to-peak potential causes strong capacitive coupling of RF power into the plasma generated within the tube 102. The strong capacitive coupling of RF power into the plasma is undesirable because it reduces the RF energy being inductively coupled to the plasma as intended by the coil 104. Capacitive coupling generally presents a large sheath voltage near the dielectric tube. The high voltage near the tube 102 causes significant erosion of the interior surface of the tube 102 as ions from the plasma are accelerated by the large sheath voltage to impinge on the dielectric tube material. The erosion on the tube 102 reduces the useful life of the tube and leads to contaminant generation during processing that may cause defects on substrates. The erosion also reduces the useful life of the tube as well as the remote inductively coupled plasma source, which results in additional costs and processing down-time for repairs and/or replacements. Furthermore, it is desirable to reduce or minimize the capacitive coupling because minimizing the capacitive coupling generally leads to a higher density plasma for a given power.
Therefore, there is a need for a remote inductively coupled plasma source that maintains a low coil voltage in the vicinity of the plasma tube, thereby reducing the capacitive coupling between the coil and the plasma and the erosion from the internal surfaces of the plasma tube.